Molecular sieves are a class of important materials used in the chemical industry for processes such as gas stream purification and hydrocarbon conversion processes. Molecular sieves are porous solids having interconnected pores of different sizes. Molecular sieves typically have a one-, two- or three-dimensional crystalline pore structure having pores of one or more molecular dimensions that selectively adsorb molecules that can enter the pores and exclude those molecules that are too large. The pore size, pore shape, interstitial spacing or channels, composition, crystal morphology and structure are a few characteristics of molecular sieves that determine their use in various hydrocarbon adsorption and conversion processes.
For the petroleum and petrochemical industries, the most commercially useful molecular sieves are known as zeolites. Zeolites are metallosilicates having an open framework structure formed from corner sharing the oxygen atoms of [SiO4] and other metal oxides such as [AlO4]tetrahedra. Mobile extra-framework cations reside in the pores for balancing charges along the zeolite framework. These charges are a result of substitution of a tetrahedral framework cation (e.g., Si4+) with a trivalent or pentavalent cation. Extra-framework cations counter-balance these charges preserving the electroneutrality of the framework, and these cations are exchangeable with other cations and/or protons.
In principle, there are two routes leading to the formation of a particular molecular sieve structure with a particular framework composition, e.g., a particular metallosilicate such as an aluminosilicate of the same crystal structure: (1) direct synthesis and (2) post-synthetic treatment (secondary synthesis). Direct synthesis is the primary route for the synthesis of molecular sieves.
Depending on the nature of the molecular sieves and the chemistry of their formation, some of these molecular sieves can be synthesized using a broad spectrum of framework compositions, e.g., an all-silica form, an aluminosilicate form, and a borosilicate form, whereas the synthesis of other structures succeeds only if certain heteroatoms (e.g., boron) are present in the synthesis mixture and, in turn, incorporated into the framework.
Molecular sieves identified by the International Zeolite Associate (IZA) as having the framework structure code SFS are known. SSZ-56 is a known crystalline SFS material, and is useful in many processes, including various catalytic reactions. Borosilicate SSZ-56 (B-SSZ-56) and methods for making it are disclosed in U.S. Pat. No. 7,226,576 and by S. Elomari et al. in Microporous Mesoporous Mater. 2009, 118, 325-333. Borosilicate zeolites, however, are not sufficiently catalytically active to be practicable for certain hydrocarbon conversion processes. Moreover, the SDA used to prepare borosilicate SSZ-56, the trans isomer of N,N-diethyl-2-methyldecahydroquinolinium cation, requires extensive purification as the cis and trans isomers of this molecule have completely different phase selectivities.
To date, attempts for direct synthesis of aluminosilicate SSZ-56 have not been successful.
Accordingly, there exists a need for a method of directly preparing aluminosilicate SSZ-56.